Here is my 2 cents worth.
The question about how long a sail lasts is a hard one to answer, I guess you can kind of relate it back to car tires, it depends on the amount of usage, type of usage and quality of the initial purchase.
Tires will last for a long time before they are actually to worn out to get you from A-B anymore, but your performance will be down, and they could blow out at any stage. Even when they fail a warrant they still have a year or so of use out of them before they are completely ruined.
For sails its even harder to pin point how long they will last as there as so m